Use of ultrasound in emergency medicine: indications, benefits, and pitfalls
THE USE OF ULTRASOUND IN EMERGENCY MEDICINE: INDICATIONS, BENEFITS, AND PITFALLS
Soren Boysen DVM, DACVECC; Serge Chalhoub DVM, DACVIM (SAIM);
Faculty of Veterinary Medicine, University of Calgary
Calgary, AB Canada
Veterinary Point of Care Ultrasound (VPOCUS) now includes multiple ultrasound- techniques that allow practitioners’ to rapidly assess patients for underlying conditions, often life-threatening, without compromising patient safety. They are specifically designed to detect injury within the abdomen, thorax and pericardial space including free abdominal fluid, pneumothorax, pleural effusion, general lung pathology, basic cardiac pathology, and assess intravascular volume status within minutes of patient arrival. They are extremely valuable in trauma patients, unstable emergency patients, daily assessment of critically ill patients, and for general patient evaluation. The information provided by these exams is instrumental in the management of these patients and they can be implemented into everyday practice. It is important to note that VPOCUS exams are not extensive abdominal or thoracic ultrasound nor are they echocardiograms. They are point of care rapid ultrasound techniques that are performed at the same time as the initial patient evaluation and treatment (physical exam, blood pressure, IV catheter, IV fluids, sedation, analgesia, SPO2, minimum emergency database) or as part of continued daily patient monitoring. They are repeatable and objective, and findings are often answered by simple binary questions. They are validated, evidence-based, sensitive and specific, and take under 10 minutes to complete. Indications to VPOCUS exams include, but are not limited to:
Any small animal trauma patient, blunt or penetrating, particularly those that are unstable, that have a total solids less than 60 g/L and/or a decreased PCV, or that show external injury
Any small animal patient presenting with unstable cardiovascular or respiratory signs, particularly if the underlying cause is uncertain
Any patient in which pericardial effusion is suspected (pulses paradoxes, muffled heart sounds, electrical alternans)
Any patient suspected to have pneumothorax (dyspnea with decreased breath sounds dorsally)
Any patient suspected to have pleural effusion (dyspnea with decreased breath sounds ventrally)
Any patient in which intra-abdominal free fluid is suspected
Any collapsed and/or unstable patient (i.e. elevated shock index, hyperlactatemia, unexplained hypotension, tachycardia, or decreased mentation) regardless of trauma, particularly if the underlying cause is uncertain.
Any patient with acute abdomen/abdominal pain
Post-surgical patients that become unstable or in whom there is a concern for bleeding or risk of dehiscence/peritonitis
Some keys points:
Veterinary point of care ultrasound (VPOCUS) exams cannot replace a physical exam and are in fact often guided by the initial findings of the triage exam (pulses paradoxes, shock, respiratory distress, abdominal pain and vomiting, muffled lung sounds etc.)
They often provide complimentary information which in many situations directs further direct diagnostics and therapies that may be lifesaving
Do the entire VPOCUS scan but answer the most urgent lifesaving question(s) first!
BRING THE MACHINE TO THE PATIENT! Don’t discontinue stabilization efforts to perform VPOCUS.
A key approach to learning and expanding the role of VPOCUS is to get comfortable asking yes/no binary questions.
By clearly defining the objectives of the rapid ultrasound, one can avoid “fishing expeditions” that are often associated with low pre-test probabilities and can lead to significant increases in the likelihood of false positive results
Human studies show the likelihood of false negative and false positive results are markedly decreased when asking binary questions
Do a complete thorough POCUS assessment to answer the binary question being asked (if you are looking for abdominal fluid, do a complete fluid search at each site you evaluate!)
The FAST abdominal exam, described in 2004 (Boysen et al 2004), was the first VPOCUS exam to be validated in small animals. The goal was to detect free peritoneal fluid following blunt abdominal trauma, and therefor concentrated on 4 key sites of the abdomen; sites where target organs were most likely to be injured following trauma; liver, spleen, kidneys and urinary bladder, and where fluid is most likely to accumulate based on patient positioning and gravitational forces. The study demonstrated that this FAST abdominal protocol was sensitive and specific for the detection of free abdominal fluid. The study also demonstrated that abdominal FAST can be performed during resuscitation, was rapid (<5 minutes), required minimal experience, was repeatable, and was noninvasive. Abdominal VPOCUS has now been validated in non-trauma cases (McMurray, Boysen, Chalhoub; JVECCS 2016). How accurate is abdominal VPOCUS?
The detection of free abdominal fluid via sonography is more sensitive than radiographs
A recent study by Walters (JVECC 2018), compared the original 2004 Abdominal FAST to CT for detection of free fluid by minimally trained ER docs and found excellent agreement (Kappa 0.82)
Although abdominal VPOCUS localizes fluid to the abdominal cavity, which permits centesis and fluid analysis, it cannot identify the actual abdominal organ injured in most cases (contrast enhanced ultrasound not done much in veterinary medicine)
Limitations: penetrating trauma and retroperitoneal injury have lower sensitivity for finding effusion, trauma does not always produce effusion and sometimes there is a delay in the appearance of effusion (hence why serial exams are recommended).
Thoracic VPOCUS: Lung, focused Heart and Pleural Space
Arguably, patients presenting with respiratory distress can be quite challenging as it is not always easy to differentiate cardiac, pleural space and parenchymal disease, particularly in cats. An incorrect diagnosis may result in life threatening interventions being delayed, or lead to an incorrect therapy being administered, which may cause patients to deteriorate. There are several algorithms that have been developed to help differentiate cardiac from non-cardiac causes of respiratory distress, most of which rely on radiographs and a cardiology consult if the patient is sufficiently stable, and/or physical exam findings and history of the patient is unstable. Most algorithms unfortunately do not incorporate the use of point of care ultrasound by non-specialists in differentiating causes of respiratory distress in cats or dogs. The skills required to perform pleural space and lung point of care ultrasound are easily learned with minimal formal training and can differentiate the major causes of respiratory distress. A particular advantage of pleural space and lung ultrasound is the fact it can be performed while the patient is receiving oxygen therapy, anxiolytics, and other stabilization efforts. In general, if it’s possible to auscult the patient with a stethoscope, thoracic VPOCUS can also be performed, even in an oxygen cage if necessary.
Following the original TFAST study, additional thoracic VPOCUS techniques have been developed with different objectives. A study by Rademacher et al (2014 Vet Rad Ultrasound) developed a lung ultrasound protocol which was the first to demonstrate that alveolar interstitial syndrome (AIS) can be diagnosed in dogs using sonography. Subsequently multiple VPOCUS techniques (Ward et al, JAVMA 2017; Lisciandro et al, 2014 Vet rad Ultrasound; Vezzosi et al, 2017 JVIM; Armenise A, Rudloff E, Boysen SB et al, JVECC 2017 in press) have been used for the detection of AIS.
In addition to advancements in detecting lung pathology, thoracic VPOCUS can also detect underlying cardiac function abnormalities in cats and dogs. Recent studies clearly show cardiovascular POCUS performed by non-specialists helps to differentiate respiratory from cardiac causes of dyspnea in both cats and dogs (Ostroski C et al, JVECC 2016 abstract; Hezzell MJ et al, JVIM 2017 abstract, in press). Finally, thoracic VPOCUS has recently been demonstrated to help detect intravascular volume changes in dogs and cats via assessment of the caudal vena cava (references)
With so many thoracic protocols being used in small animals there is some confusion as to what clinicians mean when they state “I did a TFAST exam” or “I assessed the thorax with sonography”. It is therefore important to standardize the approach to thoracic VPOCUS (e.g. searching for pleural effusion, pericardial effusion, pneumothorax, basic cardiac function, volume status, etc.) so that the information stays objective and translatable. One approach to solving the confusion surrounding the ever-expanding exams incorporated into VPOCUS is to return to the binary questions VPOCUS was originally designed to answer (pleural fluid yes/no, pneumothorax yes/no etc.). This approach helps in keeping these exams standardized, as well as answering important clinical questions (hence why we do these exams).
In the thorax, the broad clinically relevant questions we ask include:
Is there pneumothorax (is there a glide sign or B lines)?
Is there pleural effusion?
Lung: Is there AIS (are there an increased number of B lines)?
Is there pericardial effusion?
Is there adequate contractility (decreased)?
Is there left atrial enlargement (subjective left atrial aortic root ratio enlargement)?
Caudal vena cava:
Is there evidence of caudal venal caval distention?
Does the caudal vena cava decrease in size during inspiration?
Limitations: certain normal and abnormal artefacts (z lines, e lines) can be confused for b-lines, a glide sign can be difficult to detect, pneumothorax can be challenging to rule in, small amounts of effusion can be missed if not careful, steeper learning curve for the heart).